d, showing (1) the layer of non-striated cells. _B._ Three
non-striated cells highly magnified.

*Work of the Non-striated Muscles.*--The work of the non-striated muscles,
both in purpose and in method, is radically different from that of the
striated. They do not change the _position_ of parts of the body, as do
the striated muscles, but they alter the _size_ and _shape_ of the parts
which they surround. Their purpose, as a rule, is to move, or control the
movement of, materials within cavities and tubes, and they do this by
means of the _pressure_ which they exert. Examples of their action have
already been studied in the propulsion of the food through the alimentary
canal and in the regulation of the flow of blood through the arteries
(pages 159 and 49). While they do not contract so quickly, nor with such
great force as the striated muscles, their work is more closely related to
the vital processes.

*Structure of the Heart Muscle.*--The cells of the heart combine the
structure and properties of the striated and the non-striated muscle
cells, and form an intermediate type between the two. They are
cross-striped like the striated cells, and are nearly as wide, but are
rather short (Fig. 112). Each cell has a well-defined nucleus, but the
sarcolemma is absent. They are placed end to end to form fibers, and many
of the cells have branches by which they are united to the cells in
neighboring fibers. In this way they interlace more or less with each
other, but are also cemented together. They contract quickly and with
great force, but are not under control of the will. Muscular tissue of
this variety seems excellently adapted to the work of the heart.

[Fig. 112]

Fig. 112--*Muscle cells from the heart*, highly magnified (after Schaefer).

*The Muscular Stimulus.*--The inactive, or resting, condition of a muscle
is that of relaxation. It does work through contracting. It becomes
active, or contracts, only when it is being acted upon by some force
outside of itself, and it relaxes again when this force is withdrawn. Any
kind of force which, by acting on muscles, causes them to contract, is
called a _muscular stimulus_. Electricity, chemicals of different kinds,
and mechanical force may be so applied to the muscles as to cause them to
contract. These are _artificial_ stimuli. So far as known, muscles are
stimulated _naturally_ in but one way. This is through the n